Next generation cellular communications systems including the Long Term Evolution (LTE) system currently being standardized by the Third Generation Partnership Project (3GPP) support higher data rates than existing cellular communications systems such as the Universal Mobile Telecommunications System (UMTS) and the Global System for Mobile communications (GSM). The higher data rates require a larger frequency bandwidth for each physical channel. While GSM systems use a channel bandwidth of 0.2 MHz, UMTS systems have a channel bandwidth of already 5 MHz, and next generation systems will support channel bandwidths of up to 20 MHz and more.
The LTE standard for example defines a maximum channel bandwidth of 20 MHz which can be adjusted in steps of 1.25 MHz. The feature of an adjustable channel bandwidth increases the usage flexibility with respect to available spectrum resources and additionally permits a smooth migration from GSM and UMTS systems. During the migration phase it may be desired to use the radio frequency spectrum currently assigned to GSM and UMTS systems also by LTE systems. The resulting co-existence of multiple cellular communications system within a limited radio frequency spectrum gives rise to new challenges.
System operators may want to fill in the co-existence phase unused portions of their spectrum resources originally allocated to GSM or UMTS services with high data rate LTE services. According to the current allocation schemes, this filling can be done with a bandwidth granularity of 180 kHz at a frequency grid size of 100 kHz. Given such a high bandwidth granularity and small frequency grid size, the filling may lead to situations in which a narrow band signal (e.g. a GSM signal) comes very close to an LTE band edge. Consequently, GSM signal interferences in the LTE band may occur.
As a result of such interference from GSM signals, the LTE receiver performance will be degraded. The specific amount of receiver performance degradation in the presence of an interference signal depends on the blocking capabilities of the receiver. The receiver blocking capability is a measure of the receiver ability to receive a desired signal at its assigned frequency in the presence of interference on frequencies other than those of the adjacent channels. Such interference is sometimes also referred to as in-band blocking.
In-band blocking resulting, for example, from a GSM signal being closely located to an LTE band edge is difficult to filter out using an analog receiver filter due to the large and potentially variable signal bandwidths of up to 20 MHz. Even filters having a switchable bandwidth capability are not useful here because the typical switching granularities are by far not sufficient to cope with all possible co-existence scenarios. If, for example, the filter bandwidth can be selected in steps of 5 MHz and the LTE signal bandwidth is around 6 MHz, the filter bandwidth may be set to 10 MHz. Such a filter setting will, however, leave laterally 2 MHz on each side of the signal spectrum where blocking signals are not filtered at all. To cope with blocking signals in such situations, analog-to-digital converters (ADC) with an extremely high resolution of possibly more than 16 bit are needed. However, ADCs having such a high resolution and additionally providing the required sample rates of more than 30 mega samples per second (Msps) are currently not available.
EP 1 156 589 A teaches an approach for processing a Digital Radio Mondial (DRM) intermediate frequency (IF) signal in a situation in which an IF bandpass filter bandwidth (of e.g. 20 kHz) is larger than the signal bandwidth (of e.g. 10 kHz). In such a situation, the oscillation frequency of a down-conversion stage preceding the IF filter may be detuned such that an unwanted signal portion behind one of the DRM signal edges lies outside the passband of the IF filter. Since DFM systems use a two-step down-conversion, the detuning and bandpass filtering steps may be repeated in further down-conversion and filter stages to additionally remove an unwanted signal portion behind the other DRM signal edge.
The approach taught in EP 1 156 589 A pertains to an IF signal processing stage and can therefore not be applied to baseband signals. Moreover, baseband processing does not involve a two-step down-conversion, and it is therefore evident that performing only one of the two filtering operations proposed in EP 1 156 589 A would not be helpful to remove interference components behind both signal edges of a (complex-valued) baseband signal.